1. Field of the Invention
The field of the invention pertains to organic resin-coated proppants for use in fracturing subterranean geological formations to stimulate oil and gas production.
2. Description of the Related Art
Hydraulic fracturing, or “fracking” is a now old technique which has been used to stimulate oil and gas recovery. A fluid containing solid particles is injected into the oil- or gas-bearing (or both) formation to fracture the rock in the hydrocarbon-bearing zone. By such means, “clear” passages are created which allow the hydrocarbon and associated water, if present, to flow more rapidly to the well bore. This technique is becoming more widely used in shale formations where vertical movement through shale layers is very low.
Proppants are used in the fracturing fluid to ensure that the fractures remain open to flow. For this purpose, coarse sand and gravel have been used, as well as ceramic proppants. Unfortunately, these are somewhat brittle materials which can be crushed at the high pressures of the subterranean formations, thus reducing the fracture width as well as generating fines which can plug open spaces between remaining proppant particles.
Thus, fracture conductivity may be reduced by small proppants or fines. In fracture conductivity testing using proppants confined between sandstone cores, embedment of proppant into the core is frequently observed after exposure to elevated stress. In the process of embedment, spalling of fines from the rock is displaced into the proppant pack. Proppant pack conductivity damage from embedment results in loss of proppant pack width as the proppant embeds into the rock and proppant pack pore throats are plugged by displaced formation fines. The pack permeability is thereby reduced.
A second source of fines results from proppant crushing. Such fines are generated at the fracture-face to proppant pack interface as in situ closure stresses acting upon the fracture cause failure of the proppant, the formation rock, or both. Such stresses may cause the proppant to be compressed together such that fines are generated from the proppant pack and/or reservoir matrix. Further, fines composed of formation material (e.g., shale, sand, coal fines, etc.) may present similar problems and may be produced, for example, within the fractured formation due to stresses and forces applied to the formation during fracturing.
Proppant packs containing both sand and a deformable polymer proppants substantially reduce proppant crushing. Such proppant packs are disclosed in U.S. Pat. Nos. 6,059,034 and 6,330,916. In addition to sand, such proppant packs contain deformable additives which act as a cushion and minimize the point stresses applied to the proppant and limit crushing of the sand. However, at elevated stress levels, the permeability and porosity levels of such proppant packs are compromised by embedment and spalling. The proppants used in these references consist of a traditional proppant, i.e. sand, and a deformable particulate material such as polystyrene/divinylbenzene beads.
A further concept for improving the conductivity of proppants is to coat the proppant particles with an organic resin. In U.S. Pat. No. 3,929,191, sand or beads are coated with a fusible, i.e. thermoplastic (non-crosslinked) phenolic resin and injected into the geological formation. The phenolic resin then crosslinks in situ to an infusible state, and agglomerates of the proppants also form as a result. In the process for making such proppants, it is disclosed to employ a very minor amount of aminoalkylalkoxysilane as a coupling agent. However, the small amount of coupling agent used cannot form a silicone resin, and is used only to increase adherence of the phenolic resin to the sand particles. Phenolic resin coated proppants have been widely used, but suffer from the disadvantage that they continue to crosslink under the harsh subterranean conditions, becoming brittle. They may then fracture, generating the problems addressed previously.
In U.S. Pat. No. 7,883,740, it is proposed to coat proppant particles with a two layer coating, the first layer of a curable resin which is allowed to cure, followed by deposition of a second layer which is also allowed to cure. The two layers may be formed of the same curable resin. Such particles have the unwanted characteristic of shedding their outer layer, generating deformable fines which can substantially reduce porosity.
U.S. Pat. No. 7,322,411 discloses forming deformable proppants by coating proppant particles with a deformable polymer. Preferred polymers are phenol/formaldehyde resins, melamine/formaldehyde resins, and polyurethane resins. The formaldehyde-based resins have a tendency, as explained earlier, to continue to crosslink and ultimately become brittle. Polyester polyol-based polyurethane resins are unstable with respect to hydrolysis, and both these and the more hydrolysis-stable polyether-based polyurethanes tend to be expensive.
In U.S. published application 2003/0186820 A1, an in situ method of producing elastomer coated proppants is disclosed, wherein a silicone elastomer-forming component in liquid form is injected into the formation together with the particulate proppant, and polymerization within the formation enables the formation of a flexible and coherent mass. The varied underground conditions create problems with such an approach, and if too elastomeric, the gel-like masses can plug the formation rather than creating the desired hydrocarbon flow paths. Moreover, the curable silicone elastomer precursors are expensive.
It would be desirable to provide proppants which are deformable and whose conductivity is thus long lasting, while maintaining the benefits of the ease of production and cost-effectiveness of phenolic coated proppants.